In recent memory hard disk drives, high storage capacity and miniaturization have been demanded. In order to increase the recording density, it has been strongly urged to lower flying height of a magnetic head and to reduce the unit recording area. Along with this trend, the surface quality required after polishing has become severely assessed every year even in a method for manufacturing a substrate for memory hard disk, so that the sizes and depths of scratches and pits allowed in order to meet the requirements of the reductions in the surface roughness, the average waviness, the roll-off, and the projections and the decrease in unit area have been becoming increasingly small.
In order to meet such requirements, there has been proposed a polishing composition for an aluminum disk substrate containing an abrasive containing plural colloidal silica particles having different mono-modal number particle size distributions, capable of obtaining an aluminum disk substrate having small average waviness and few surface defects (see, for instance, Japanese Patent Laid-Open No. 2002-30274).
However, in the above-mentioned polishing composition, while the average waviness is reduced, it is unsatisfactory in the reduction in microwaviness (short wavelength (50 to 500 μm) and long wavelength (500 μm to 5 mm)), which takes an intermediary wavelength of the roughness and the average waviness which have been recently considered to be important in the lowering of the flying height of a magnetic head. Therefore, further improvements have been desired.